Merge branch 'kvm-redo-enable-virt' into HEAD

Register KVM's cpuhp and syscore callbacks when enabling virtualization in
hardware, as the sole purpose of said callbacks is to disable and re-enable
virtualization as needed.

The primary motivation for this series is to simplify dealing with enabling
virtualization for Intel's TDX, which needs to enable virtualization
when kvm-intel.ko is loaded, i.e. long before the first VM is created.

That said, this is a nice cleanup on its own.  By registering the callbacks
on-demand, the callbacks themselves don't need to check kvm_usage_count,
because their very existence implies a non-zero count.

Patch 1 (re)adds a dedicated lock for kvm_usage_count.  This avoids a
lock ordering issue between cpus_read_lock() and kvm_lock.  The lock
ordering issue still exist in very rare cases, and will be fixed for
good by switching vm_list to an (S)RCU-protected list.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 09126bb8cc9f..1b52b1b7bbc4 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2648,6 +2648,23 @@
 
 			Default is Y (on).
 
+	kvm.enable_virt_at_load=[KVM,ARM64,LOONGARCH,MIPS,RISCV,X86]
+			If enabled, KVM will enable virtualization in hardware
+			when KVM is loaded, and disable virtualization when KVM
+			is unloaded (if KVM is built as a module).
+
+			If disabled, KVM will dynamically enable and disable
+			virtualization on-demand when creating and destroying
+			VMs, i.e. on the 0=>1 and 1=>0 transitions of the
+			number of VMs.
+
+			Enabling virtualization at module lode avoids potential
+			latency for creation of the 0=>1 VM, as KVM serializes
+			virtualization enabling across all online CPUs.  The
+			"cost" of enabling virtualization when KVM is loaded,
+			is that doing so may interfere with using out-of-tree
+			hypervisors that want to "own" virtualization hardware.
+
 	kvm.enable_vmware_backdoor=[KVM] Support VMware backdoor PV interface.
 				   Default is false (don't support).